Element Selenium, Se, Non Metal
|From comparatively concentrated solutions, selenium is obtained as a bright red precipitate which, even under the influence of the temperature of boiling water, cakes together to a dense, black-red mass. At 217° selenium melts and forms a dark, viscous liquid which solidifies, on being quickly cooled, to an amorphous mass of a black-red colour, which breaks with a conchoidal fracture. If this amorphous selenium be kept some time at a temperature of 100° to 150°, it becomes crystalline and grey, with a somewhat metallic lustre. At 650° it boils. |
Whereas the amorphous selenium does not conduct the electric current to a measurable extent, the property of conductivity is possessed by the crystalline form.
In this connection the special peculiarity is met with, that the electrical conductivity of crystalline selenium depends on the illumination which it experiences. Its Conductivity is all the greater the stronger the light which it receives. On altering the strength of the light, these changes occur in an exceedingly short time, and have, therefore, found practical application to the electrical transmission of signals. It has not yet been determined on what this peculiarity depends, but it appears that traces of foreign substances, more especially of the selenium compounds of the heavy metals which are mixed with selenium, play a great part.
In its compounds, selenium greatly resembles sulphur, for hydrogen selenide, selenious acid, and selenic acid are not only analogously constituted to the corresponding sulphur (compounds, but, in part also, have a similar behaviour. From the special description of the different substances, these similarities and differences will become apparent. The combining weight of selenium is Se = 79.2.
Hydrogen selenide, H2Se, is a colourless gas with a very unpleasant smell, recalling that of decaying radish. It is very poisonous, and calls for special care in working with it. It readily dissolves in water in large amount. The solution has a feebly acid reaction, and in contact with the air red selenium quickly separates out, the hydrogen of the seleniuretted hydrogen combining with the oxygen of the air to form water - 2H2Se + O2 = 2H2O + 2Se.
Hydrogen selenide is the hydrogen acid of selenium in the same sense as hydrogen sulphide is the hydrogen acid of sulphur; its aqueous solution contains the ions Se'' and HSe', both of which are colourless. The salts of hydrogen selenide are the metallic selenides. By reason of the two combining weights of hydrogen, seleniuretted hydrogen is dibastic, and its salts, therefore, contain either two combining weights of a monovalent metal or one combining weight of a divalent one, etc. Seleniuretted hydrogen can be obtained by the actions of stronger acids on metallic selenides. The method usually pursued is to prepare ferrous selenide, FeSe, by heating selenium with iron filings, and to treat this with hydrochloric acid. In a manner perfectly analogous to the preparation of sulphuretted hydrogen, ferrous chloride and seleniuretted hydrogen are formed in accordance with the equation
FeSe + 2HCl = H2Se + FeCl2.
From the solutions of the different heavy metals, seleniuretted hydrogen precipitates the corresponding selenium compounds as red or dark coloured substances. The compounds of selenium with sodium and similar metals are, however, readily soluble in water. In the air, these decompose similarly to seleniuretted hydrogen, so that after some time a red precipitate of selenium is deposited from their solutions. This property is made use of for the purpose of obtaining or of purifying selenium. The crude material is fused with sodium hydroxide (or with sodium carbonate, which acts in a similar manner), and the filtered solution of the melt is exposed in shallow vessels to the air.
The poisonous action of seleniuretted hydrogen mentioned above, depends essentially on its ready decomposability by oxygen. Selenium thereby separates out in the tissues in a finely divided form, and acts both chemically and mechanically as an irritant.
|Selenium was first identified in 1817 by the Swedish chemist Jakob Berzelius (1779-1848) after analyzing an impurity that was contaminating the sulfuric acid (H2SO4). Berzelius and his colleague Johann Gottlieb Gahn (1745-1818) were studying a method of producing sulphuric acid in lead cameras "We observed residues of a substance, partially red and partially light-brownish, with a very intense scent in the bottom of the camera. Such scent, according to Klaproth, is the sign of presence of tellurium, so at first we thought it was tellurium. Gahn noticed that in mines of Falun, were sulphur is collected, similar scent of tellurium is also often felt. Interested and hoping to discover a new metal, I commenced to investigate substance of the residues. However, more careful attempts to separate tellurium, comparing with the analysis of burnt ores from Falun, revealed that there were no residues of this element, in spite of its identical properties. It was a new substance and, following the identity of properties, was given the name Selenium, term that derives from the Greek ∑εληνη (Moon), since Klaproth had named tellurium for the Earth."|
|Average crustal abundance is 1.4x10-5% by mass, in sea waters 4x10-3mg/l. Over 50 selenium minerals are known such as native selenium, berzelianite (copper selenide), naumannite Ag2Se, chalcomenite CuSeO3 x 2H2O. Selenium concentration in pyrite (iron sulphide), galena (lead sulphide), bismuthinite (bismuth sulphide) and other sulphides, as well as in volcanic sulphur, reaches several per cents.|
Selenium is most commonly produced from selenide in many sulphide ores, such as those of copper, Silver, or lead.
Selenium is a very rare trace element. It history in the Earth's crust is closely related to sulphur history. Selenium has concentration ability and, despite the low abundance, creates 38 minerals, including selenide, selenate and selenite. Isomorphic impurities of selenium in sulphides and native sulphur are typical.
Selenium energetically migrates in biosphere. Rocks, volcanic fumes and thermal waters are the main source of selenium concentration in living organisms. For this reason soils of contemporary and ancient volcanic areas are enriched by selenium, reaching in clays and slates up to 6 x 10-5%. In 1952 Abdullaev, the young Soviet scientists, who later became the President of Azerbaijan Academy of Science, noticed, that the spectral sensibility of human eye matched with spectra of selenium used in photoelectric cells. Thus hypothesis arose that selenium has its role in light energy transformation into the electrical signals energy, which is responsible for vision.
Long time later this hypothesis had been confirmed by discovering selenium in retina. The concentration in human eye is 7 µg, 100 times less than in eyes of sharp-sighted eagle, 780 µg. This discovery was proved in experiments with rabbit's vision.
Selenium has a biological role also in enzymatic reaction; its compounds are responsible for protection from irradiation injury. Selenium needs level is, however, not so high, approximately 50...100 µg per kg of food.